1. Field of the Invention
The present invention relates to a laser light radiation device used in an image forming apparatus adopting an electrophotography process and, more particularly, to a laser light radiation device which is used in, e.g., a full-color printer, and has a light source for emitting laser light.
2. Related Background Art
In conventional image forming apparatuses, as a high-speed, low-noise printer, a laser beam printer which adopts an electrophotography process is known.
A typical application of the laser beam printer is binary recording for forming an image such as a character, a figure, or the like on a photosensitive member depending on the radiation/non-radiation state of a laser beam. In general, since recording of a character, figure, or the like does not require any halftone expression, the printer structure can be simplified.
However, even in such a binary recording method, demand has arisen for a printer which can achieve a halftone expression. As a method of achieving a halftone expression, a method adopting a dither method, a density pattern method, or the like is known. However, as is well known, a printer which adopts a dither method, a density pattern method, or the like cannot provide a high resolution. In recent years, a method of forming halftone pixels while maintaining a high resolution without decreasing the recording density has been proposed. This method is a PWM method for performing halftone pixel formation by modulating the pulse width of a laser beam to be radiated in accordance with an image signal. According to this PWM method, a high-resolution, multi-gradation level image can be formed, and hence, this method is indispensable for a color image forming apparatus which requires both the high resolution and multi-gradation level characteristics. More specifically, the PWM method can achieve area gradation of a dot formed by a beam spot in units of pixels, and a halftone expression can be realized without decreasing the pixel density (recording density) to be recorded.
FIG. 8 shows this prior art. FIG. 8 is a schematic view of a laser beam scanner portion. A laser drive circuit 101 is driven by a signal transmitted from an image information signal source (not shown) to scan the laser beam position by a laser beam scanner using a semiconductor laser unit 102, a collimator lens 103, a polygonal mirror 104, and an f-.theta. lens 105. Thus, a beam spot is focused on the surface of a photosensitive drum 110 as an image carrier to form an electrostatic latent image on the surface of the drum 110. Thereafter, image recording is realized on a recording sheet via a known electrophotography process.
As described above, with the PWM method, a high-resolution image can be formed.
However, since the PWM method reduces an image to be formed in one pixel by narrowing the pulse width, the amount of laser light to be radiated onto one pixel (laser intensity) becomes extremely small. Therefore, as the resolution becomes higher, a desired exposure amount cannot be obtained, and the density of an image decreases.
For this reason, the resolution of an image may be increased by reducing the spot size of a laser beam itself. However, the obtainable reduction in spot size is limited in a method of focusing the spot size using a lens.